(a) Field of the Invention
The present relates to a vari-focal lens system for cameras, and more specifically to a vari-focal lens system for video cameras.
(b) Description of the Prior Art
In the recent years where compact, light-weight video cameras are offered at low prices, demand for domestic video cameras is increasing rapidly. In order to further develop this demand, it is necessary to design compactor video cameras having lower weight and manufacturable at lower cost, thereby posing problems to obtain more compact lens systems having lower weight and manufacturable at lower cost.
Currently, lens systems for domestic video cameras are designed mainly as zoom lens systems having a zoom ratio of 6 and an F number of F/1.2 to F/1.6. This is because these specifications are very advantageous for lens design and adequate for meeting practical needs. As such conventional zoom lens systems for video cameras, there are known the lens systems disclosed by Japanese Unexamined Published Patent Applications No. 153913/58 and No. 24213/62.
The zoom lens system disclosed by the former patent consists of four lens units, out of which the second lens unit consists of three lens components, whereas the third and fourth lens units consist of eight lens components in total.
The zoom lens system disclosed by the latter patent consists of four lens units, out of which the second lens unit having a vari-focal function consists of three lens components, whereas the third lens unit having a function to correct deviation of image point caused due to variation of focal length and the fourth lens unit having an imaging functions consist of four to seven lens components in total.
Either of these conventional zoom lens systems has a defect that it comprises a large number of lens components, concretely 10 to 14 lens components, requires high manufacturing cost accordingly, and is large and heavy in weight.
Further, in the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 110112/61, the second lens unit is composed of two lens components by using multiple a spherical surfaces, and the third lens unit having a function to correct deviation of image point caused due to variation of focal length and a fourth lens unit having an imaging function are composed of four lens components in total. Owing to this design, the lens system as a whole consists of a small number of lens components, concretely eight lens components. However, this zoom lens system requires high manufacturing cost since it adopts multiple cemented surfaces having high curvature. Moreover, the cemented doublet arranged in the second lens unit has very high curvature on the surface thereof and allows lateral chromatic aberration to be abruptly aggravated at the marginal portion of image, thereby lowering contrast and producing remarkable color separation at the marginal portion of image.
U.S. Pat. No. 3972592 discloses a zoom lens system consisting of three lens units, out of which the second lens unit consists of a single biconcave lens component and the third lens unit is kept fixed during variation of focal length. Accordingly, this zoom lens system is different from the vari-focal lens system according to the present invention in which a third lens unit is shifted wholly or partially for correcting deviation of image point during variation of focal length as described later.
Furthermore, the zoom lens system disclosed by U.S. Pat. No.. 3160699 does not use an a spherical surface nor corrects aberrations sufficiently. In addition, the lens unit III.sub.2 comprises a large number of lens components and has a complicated composition.